Method and system for scanning of a transparent plate during earth observation imaging

ABSTRACT

An imaging system includes a body, a stage coupled to the body, and a focal plane array including one or more detectors and coupled to the stage. The imaging system also includes a lens assembly including an objective lens and a rear lens group. The lens assembly is coupled to the body and optically coupled to the focal plane. The imaging system further includes a transparent plate coupled to the body and optically coupled to the objective lens and the focal plane array. The transparent plate is disposed between the objective lens and the focal plane array. Additionally, the imaging system includes an actuator coupled to the transparent plate and configured to rotate the transparent plate relative to an optical axis of the imaging system.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationNo. 62/724,513, filed on Aug. 29, 2018, entitled “Method and System forScanning of a Transparent Plate During Earth Observation Imaging,” thedisclosure of which is hereby incorporated by reference in itsentireties for all purposes.

BACKGROUND OF THE INVENTION

Satellite imaging has been developed. Satellite ground velocity, whichdepends on orbit altitude, can be as high as 8 km/s. As a result of thehigh ground velocity, low earth orbit imaging applications using staringsensors experience image smearing. Thus, there is a need in the art forimproved methods and systems related to satellite imaging.

Embodiments of the invention address these and other problemsindividually and collectively.

SUMMARY OF THE INVENTION

Embodiments of the present invention relate generally to systems andmethods for image stabilization and, in particular, to reduction ofimage blur or smear using a transparent plate rotation system. Thetransparent plate rotation system can introduce a backscan to reduce orremove image smear, improve relative edge response, allow an increasedintegration time associated with an image, and/or allow severalsuccessive images to be captured of the same scene for image processing.Increasing the integration time can improve the signal to noise ratio ofthe sensor and improve the ability to detect features in the observedscene. Processing several successive frames of the same scene canfurther improve signal to noise ratio and allow for more complexalgorithms to detect more challenging features in the observed scene.Backscanning via the rotating transparent plate reduces or eliminatesthe need to incorporate additional scan mirrors or other complex andheavy mechanical systems to stabilize the image.

According to an embodiment of the present invention, an imaging systemis provided. The imaging system includes a body, a stage coupled to thebody, and a focal plane array including one or more detectors andcoupled to the stage. The imaging system can also include a lens coupledto the body and optically coupled to the focal plane array, and atransparent plate coupled to the body and optically coupled to the focalplane array and lens. The transparent plate can be disposed between thefocal plane array and the lens. Additionally, an actuator can be coupledto the transparent plate. The actuator can be configured to rotate thetransparent plate in one or more directions relative to the focal planearray. In some embodiments, the transparent plate is disposed at an exitpupil of the imaging system, but this is not required by the presentinvention and in some embodiments, the transparent plate is disposedbetween the objective lens and the exit pupil. In yet other embodiments,the transparent plate is disposed between the rear lens group and thefocal plane array. Moreover, in some embodiments the transparent platecomprises a planar optical element and is characterized by no opticalpower, whereas in other embodiments, one or more surfaces of thetransparent plate are characterized by a predetermined curvature and thetransparent plate is characterized by a non-zero optical power. Theseembodiments will be described in additional detail herein.

According to another embodiment of the invention, a method is provided.The method includes determining a travel velocity corresponding tomotion of a body of an imaging system, and determining a rotation ratefor a transparent plate of the imaging system based on the travelvelocity. The transparent plate is optically coupled to an image sensorof the imaging system and a lens of the imaging system. The method alsoincludes sending a first control signal to an actuator to rotate thetransparent plate at the determined rotation rate, and sending a secondcontrol signal to an image sensor of the imaging system to capture oneor more frames while the actuator rotates the transparent plate. Themethod further comprises determining that the transparent plate reachesa cutoff angle, and thereafter, sending a third control signal to resetthe transparent plate to an initial position.

According to a specific embodiment of the present invention, a method ofusing an imaging system comprising a focal plane array with one or moredetectors, a lens optically coupled to the focal plane array, atransparent plate optically coupled to the focal plane array and lens,the transparent plate being disposed between the focal plane array andthe lens, and an actuator coupled to the transparent plate, the actuatorbeing configured to move the transparent plate in one or more directionsrelative to the focal plane array is provided. The method includesreceiving, at a first area of the focal plane array through the lens,light from an object at a first time. The imaging system is located in afirst position relative to the object at the first time. The method alsoincludes causing the actuator to move the transparent plate in responseto movement of the imaging system relative to the object and receiving,at the first area of the focal plane array through the lens, light fromthe object at a second time. The imaging system is located in a secondposition relative to the object at the second time.

In an embodiment, moving the transparent plate comprises rotating thetransparent plate. In one implementation, the method also includesdetermining a travel velocity corresponding to the movement of theimaging system relative to the object and determining, based on thetravel velocity, a rotation rate for the transparent plate. Causing theactuator to rotate the transparent plate can include rotating thetransparent plate at the determined rotation rate. In some embodiments,the rotation rate is not constant. As an example, the rotation rate canchange based on an angle position of the transparent plate. In aspecific embodiment, the method further includes determining, based onthe travel velocity, a rotation direction for the transparent plate. Inthis specific embodiment, causing the actuator to rotate the transparentplate can include rotating the transparent plate in the determinedrotation direction. For example, the travel velocity can correspond tothe movement of the imaging system corresponds to a travel velocity ofat least one of an aircraft or a satellite. In an embodiment, the methodfurther comprises causing the one or more detectors to capture imagedata, for example, during rotation of the transparent plate. Forinstance, rotation of the transparent plate can cause light from theobject to be received at the first area of the focal plane array at boththe first time and the second time. In another embodiment, thetransparent plate can be a flat plate with no optical power or a curvedplate with non-zero optical power.

Numerous benefits are achieved by way of the present invention overconventional techniques. For example, embodiments of the presentinvention provide methods and systems that utilize controlled rotationof a transparent plate to correct for image smearing in high-velocityimaging systems, such as satellites and airplanes, thereby improvingimage quality. In some implementations, use of a transparent plateenables optical components that are small and light, thereby providing acompact package suitable for airborne or space-based platforms. Theseand other embodiments of the invention along with many of its advantagesand features are described in more detail in conjunction with the textbelow and attached figures.

BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative embodiments of the present disclosure are described indetail below with reference to the following drawing figures.

FIG. 1 illustrates three successive frames captured using an imagingsystem that does not compensate for the motion of the camera body,according to an embodiment of the present invention.

FIG. 2A illustrates an imaging system configured to backscan an imagevia rotation of a transparent plate during motion of the imaging systemaccording to an embodiment of the present invention.

FIG. 2B illustrates an imaging system configured to backscan atransparent plate by rotating the transparent plate during motion of theimaging system according to another embodiment of the present invention.

FIG. 2C illustrates an imaging system configured to backscan atransparent plate by rotating the transparent plate during motion of theimaging system according to yet another embodiment of the presentinvention.

FIG. 3 illustrates three successive frames captured to make a “snap”using an imaging system with a transparent plate that is backscanned ata velocity that corresponds to and cancels the velocity of the camerabody relative to the scene, according to an embodiment of the presentinvention.

FIG. 4A illustrates two full cycles of capturing consecutive imageframes with a transparent plate rotating at a velocity that compensatesfor the velocity of the camera body, according to an embodiment of thepresent invention.

FIG. 4B illustrates the overlap between snaps, according to anembodiment of the present invention.

FIG. 5A illustrates a focal plane array consisting of 5 stagger buttedfocal planes on a stage, according to an embodiment of the presentinvention.

FIG. 5B illustrates the ground swath width of a scan associated with 5stagger butted focal planes, according to an embodiment of the presentinvention.

FIG. 6A illustrates an example of a transparent plate laterally shiftingan incoming light ray, according to an embodiment of the presentinvention.

FIG. 6B illustrates an example of a rotating transparent plate laterallyshifting an incoming light ray, according to an embodiment of thepresent invention.

FIG. 7 is a simplified flowchart illustrating a method 700 of rotating atransparent plate to backscan an image during image collection,according to an embodiment of the present invention.

FIG. 8 is a simplified flowchart illustrating a control algorithm 800for rotating a transparent plate according to an embodiment of thepresent invention.

FIG. 9 is a simplified schematic diagram illustrating a transparentplate utilized in some embodiments of the present invention.

FIG. 10 shows a comparison of relative edge response (RER) for variousfocal plane array configurations according to embodiments of the presentinvention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Numerous sources of interference exist in satellite imagingtechnologies. To improve the signal to noise ratio of an image of ascene on the ground, individual frames of the scene on the groundcaptured by an image detector can be co-added. Co-adding is simplifiedif the image hasn't moved relative to the detector and no imageregistration is required. Problematically, large staring focal planearrays used in modern satellites and aerial cameras are subject to imageblur or smear due to the motion of the stage during the exposure time.The image blur or smear reduces the useful integration time for a largestaring focal plane array and as a result, image quality.

One existing technology to address image blur caused by the motion of afocal plane array in a moving platform, such as a satellite, includecomplex mechanical systems to physically steer the entire optical system(e.g., the lens barrel) to control the pointing of the image on thefocal plane array. Another conventional technique to stabilize an imageduring a focal plane array integration time is to incorporate faststeering mirrors in the optical chain. Fast steering mirrors are largeand can take up a significant amount of space, especially when locatedat the entrance aperture of the optical system. Because of their size,large moving mirrors require more time to move and time to settle beforethe integration time can commence, leaving less time for integration ofsignal and/or frame stacking.

Embodiments of the present invention provide an alternative method forreducing image blur or smear present in scans taken using high altitudeimaging systems, for example, satellite-based imaging systems. Morespecifically, embodiments of the present invention utilize a rotatingtransparent plate to steer the light incident on a detector. Thetransparent plate can be positioned in any suitable location along theoptical path. In some embodiments, the transparent plate is positionedin a location with the light ray bundle is smallest in diameter, suchthat the transparent plate can be small. For example, the transparentplate can be positioned at or near an exit pupil. The transparent platecan be a relatively small optical element, which therefore is easier tocontrol, faster to reset, and less disruptive than the larger movingmirrors. The rest of the optical elements within the optical system canremain fixed, as the image can be steered using only the transparentplate. Accordingly, embodiments described herein are more compact andimprove performance, efficiency, and reliability over conventionaltechniques used for spaceborne and airborne applications.

In some embodiments, the transparent plate and image detector can bepart of a satellite space platform that is moving relative to the earth.The transparent plate can be rotated within an axis normal to the axisof the detector's motion relative to the earth, such that thetransparent plate rotation opposes the detector's motion. The rotationrate and rotation direction can be matched to the travel velocity of thesatellite resulting in a backscan so that the image appears static(e.g., incident on the same area of the detector) during a focal planearray integration period.

In some embodiments, the integration period can be continuous during theduration of the backscan of the transparent plate. In other embodiments,multiple consecutive frames can be stacked together to form a singlesnap. The backscan of the transparent plate results in the target imagenot moving relative to the focal plane array during the backscan. Thetransparent plate can return to a starting position while the trailingedge of the field of view moves across a scene on the ground that hasalready been captured.

In some embodiments, the focal plane array can include a focal planearray that captures images from multiple spectral bands. Embodiments ofthe present invention provide a stabilized staring imager. The use ofthis technology enables the use of staring focal plane arrays inapplications that have a scan motion that historically used linear scansensors.

Prior to discussing specific embodiments of the invention, some termsmay be described in detail.

A “transparent plate” can include an optical element that is fullytransparent, partially transparent or translucent, or otherwisetransmissive to some or all wavelengths of electromagnetic radiation. Insome embodiments, a transparent plate can have a flat surface. Forexample, a transparent plate can have two parallel sides, such that thatthe surface of the transparent plate is not curved in at least onedirection, and such that the transparent plate has little or no opticalpower. For example, a flat transparent plate can shift the path of anincident light ray to the side without changing the angle or directionof travel of the light ray.

In other embodiments, a transparent plate can include some curvature.For example, the edges of the transparent plate can be slightly curved(e.g., either concave or convex), and can have some optical power. Theoptical power and curvature can be configured to, in conjunction withother lenses in the system, produce a uniform shift in the image acrosslarge fields of view. In some embodiments, a transparent plate caninclude two adjacent plates. A transparent plate can be composed of anysuitable materials that are optically transparent materials, such asfused silica, sapphire, diamond, Silicon, or Germanium. A transparentplate may have a high index of refraction such that incident light isshifted by a larger distance. A transparent plate can include athickness of 1-5 mm, larger than 5 mm, or any other suitable thickness.

A focal plane array can be positioned at a focal plane that can includean area where an image is in focus. The focal plane can be perpendicularto the optical axis of a lens or group of lenses. The location of afocal plane can be a property of a combination of optical elements. Insome embodiments, light detectors can be placed at or near a focalplane. In some embodiments, a focal plane array can refer to a physicalplatform in an optical system where an image is in focus, and/or one ormore light detectors within an optical system.

FIG. 1 illustrates three successive frames captured using an imagingsystem that does not compensate for the motion of the camera body. Theimaging system 100 includes a detector module 102, a stage 104, a focalplane array 106, and a lens 108. In some embodiments, the imaging system100 can be mounted in a moving vehicle such as a satellite, an aircraft,an automobile, and the like. FIG. 1 also illustrates the rays 110associated with an object on the ground 112.

In FIG. 1, the imaging system 100 is moving at a velocity 114 over theobject on the ground 112. In a first frame 116, the rays 110 associatedwith the object on the ground 112 are centered on the focal plane array106 and a first image 117 on the focal plane array 106 shows the objecton the ground 112. The motion of the system causes the image planeformed by the lens 108 to move. For a second frame 120, imaging system100 has moved relative to the object on the ground 112 due to thevelocity 114 of the imaging system 100. In the second frame 120, therays 110 associated with the object on the ground 112 are no longercentered on the focal plane array 106 but have moved a first distance122. Accordingly, a second image 121 on the focal plane array 106 isdifferent from the first image 117.

For a third frame 124, the imaging system 100 has moved further relativeto the object on the ground 112 due to the velocity 114 of the imagingsystem 100. In the third frame 124, the rays 110 associated with theobject on the ground 112 have now moved the first distance 122 and asecond distance 126. Accordingly, a third image 125 on the focal planearray 106 is different from the first image 117 and the second image121. If an image of the object on the ground 112 was produced withoutimage registration from the integration of the first image 117, thesecond image 121, and the third image 125, the integrated image of theobject on the ground 112 would include significant blur. To preventimage blur from within the first image 117, the second image 121 or thethird image 125, the integration time of the sensor must besignificantly less than the time it takes for a single pixel on thesensor to move one pixel length. Otherwise, significant image bluroccurs within each of the first image 117, the second image 121 or thethird image 125.

FIG. 2A illustrates an imaging system configured to backscan an imagevia rotation of a transparent plate during motion of the imaging systemaccording to an embodiment of the present invention. Rotating atransparent plate to backscan the image makes it possible to dwell longenough during an image to obtain a reasonable Signal to Noise Ratio(SNR). Backscanning also allows frame stacking of successively capturedframes, reducing blurring the image. If the noise is “white” in nature,frame stacking benefits the SNR by the square root of the number offrames stacked. An image made up of 4 frames stacked has 2× higher SNRthan an image taken with one frame.

The imaging system 200 can include a camera body 202, a dewar 232, acooling device 230, a detector module 206, a number of optical elements(e.g., optical elements 240, 242, 244, 246, 248, 250, 252, and 254), acontroller 208, and an I/O module 210. In some embodiments, the camerabody 202 of the imaging system 200 is positioned in a vehicle orplatform such as satellite or an aircraft. In some embodiments, thecamera body 202 can be configured to provide structural support andalignment for the detector module 206 and the optical elements. In someembodiments, the camera body can include anchor points for thecontroller 208 and the I/O module 210 and can be configured to manageheat transfer and isothermal performance. In other embodiments, thecamera body 202 can be replaced by two separate bulkheads. A firstsubset of the optical elements (e.g., 240 and 242) can be mounted on afirst bulkhead and the dewar 232 (e.g., and enclosed elements),controller 208, and I/O module 210 can be mounted on a second bulkhead.

In some embodiments, the optical elements (e.g., 240, 242, 244, 246,248, 250, 252, and 254) can include one or more lenses, filters, a beamsplitter, a collimator, a diffraction grating, and/or any other suitablecomponents. In one specific example, the optical elements can be atransparent plate 242, an objective lens 240, which can be a large outerlens, a window 244 placed at an optical opening into the dewar 232, aband-pass filter 246, a first lens 248, a second lens 250, a third lens252, and a fourth lens 254. Together, the lenses (e.g., 240, 248, 250,252, and 254) can form an inverse telephoto lens system. The band-passfilter 246 can be designed to be transparent for a specific range ofelectromagnetic radiation. For example, in some embodiments, theband-pass filter 246 may pass some or all wavelengths of infrared light.In one example, the band-pass filter 246 may allow wavelengths in therange of 2.7 μm-4.4 μm to pass.

As shown in FIG. 2A, in some embodiments, the optical elements, 246,248, 250, 252, and 254 can all be positioned inside the dewar 232, whilethe transparent plate 242 and the objective lens 240 can be positionedoutside the dewar 232. For this reason, the lenses 248, 250, 252, and254 can be referred to as inner lenses and the set of lenses 248, 250,252, and 254 can be referred to as a rear lens group. Thus, a lensassembly including objective lens 240 and the rear lens group isillustrated in FIG. 2A is provided according to embodiments of thepresent invention. In some implementations, it can be advantageous toplace optical elements inside the dewar 232, as they can thereby be keptcold to reduce background radiation. There can also be additional orless optical elements, and the optical elements shown in FIG. 2A arejust one example of possible configurations. In some embodiments, theoptical elements can be optimized for the transmission of a specificwavelength such as visible, near-infrared, short-wavelength infrared,mid-wavelength infrared, long-wavelength infrared, and far infrared.

In some embodiments, the combination of optical elements in the imagingsystem 200 can have a focal length that varies across the field of view.For example, the focal length can be 12.5 mm at the center of theoptical axis (e.g., normal to the detector module 206), and the focallength can be 46.3 mm at the edge of the observed image area. The fieldof view can cover a range of 121 degrees from one image edge to another,and the imaging system 200 can have an f-number of f/2.7. The imagingsystem 200 can be designed to image a spectral band of 2.7 μm-4.4 μm.The above parameter values are examples, and the imaging system 200 canbe configured in any other suitable manner.

In some embodiments, the transparent plate 242 can be a transmissiveoptical element with little or no optical power. Embodiments of thepresent invention do not require that the transparent plate 242 has nooptical loss, but that a suitable amount of light is transmitted asappropriate to the particular application. As an example, an opticalmaterial that is characterized by a transmittance of greater than 95%,greater than 96%, greater than 97%, greater than 98%, or greater than99%. Thus, the transparent plate 242 does not have to be characterizedby an absorbance of zero, but includes optical elements that arecharacterized by a finite but suitable transmittance. In the directionof the optical path, the transparent plate 242 may be flat andnon-curved, with an constant width dimension (e.g., along the opticalpath). A cross-sectional area of the transparent plate 242 (e.g., withina plane normal to the optical path) can have any suitable length andheight. In some embodiments, the size of the cross-sectional area can beas small as possible to minimize weight and rotational inertia, whilestill being large enough to accommodate the entire light ray bundle inthe optical path. The cross-section of the transparent plate 242 cantake the shape of a circle or oval (e.g., such that the 3-dimensionalshape is a disk), the shape of a rectangle (e.g., such that the3-dimensional shape is a box), or any other suitable shape. Embodimentsof the invention allow the transparent plate 242 to include glass and/orany other suitable material. Additionally, the transparent plate 242 caninclude two separate plates that are located adjacent to one anotherand/or directly attached.

Due to the shape and optical properties of the transparent plate,embodiments allow the transparent plate 242 to shift the location of animage on a focal plane of the detector module 206 without otherwisemodifying the shape, size, focus, or other attributes of the image. Thetransparent plate 242 can accomplish this by, for example, redirectingthe trajectory of incoming light rays as a group. The transparent plate242 may be able to shift incoming bundle of light rays to the sideand/or change the angle of the bundle without otherwise affecting thebundle at the focal plane.

The magnitude and direction of the image shift on the focal plane can bedependent on the angular position of the transparent plate 242. Forexample, if the surface of the transparent plate 242 is oriented normalto the optical path (e.g., with an angle of zero degrees), thetransparent plate 242 may have no effect or a negligible effect on theincoming light rays and resulting image. If the transparent plate 242 isinstead rotated to have an angle relative to the optical path, theincoming light rays and resulting image on the focal plane can beshifted. Since an object image will move across the focal plane as thecamera body 202 moves with respect to the earth, tilting of thetransparent plate 242 can be used to move the object image in an equaland opposite direction to thereby cancel out the movement and keep theobject image in the same location on the focal plane. In someembodiments, the transparent plate 242 can shift the image formed at thefocal plane array 218 by up to 100 microns.

Embodiments allow the transparent plate 242 to be located in anysuitable position along the optical path. For example, the transparentplate 242 can be placed in the middle of an optical system, somewherebetween the objective lens 240 (e.g., the first element that receiveslight from a scene being observed) and the last optical element 252(e.g., the last element to affect the light before it goes to the imagesensor) where there is enough space to operate the transparent platerotation. In some embodiments the transparent plate 242 can be placedwhere the light ray bundle is the smallest (e.g., has the smallestcross-section or diameter). For example, the transparent plate 242 canbe positioned at or near an exit pupil. This allows a small sizedtransparent plate 242 to be used in the imaging system 200, so as toreduce the weight and momentum of the transparent plate 242, as well asto minimize the space required by the transparent plate 242 within theimaging system. Minimizing the transparent plate 242 reduces negativeeffects, such as disturbance caused by movement of the transparent plate242, and also enables a reduction in the power and structures used forsupporting and moving the transparent plate 242. In some embodiments,the transparent plate 242 can be the limiting factor on the field ofview, and the transparent plate 242 can create the exit pupil.Additionally, the transparent plate 242 can be located outside the dewar232 so that the transparent plate 242 can be controlled withoutdisturbing the cold area.

The actuator 214 can be coupled to the transparent plate 242 and beconfigured to move the transparent plate 242 in one or more directionsrelative to the focal plane array 218. In some embodiments, moving thetransparent plate 242 may only include rotating the transparent plate242. In other embodiments, the transparent plate 242 can also be movedforward, moved laterally, or otherwise moved in any other suitablemanner. In some embodiments, the actuator 214 can include apiezoelectric actuator.

In some embodiments, the actuator 214 can be configured to move thetransparent plate 242 to counter the motion of a satellite platform oraircraft. The actuator 214 can be configured to rotate the transparentplate 242 about a single axis or multiple axes. The actuator 214 canrotate the transparent plate 242 within any suitable range of angles,such as between zero degrees (e.g., normal to the optical path) and 90degrees in any direction and about any axis. In some embodiments, theactuator 214 can provide a backscan resolution on the order of 0.1 nm.In other embodiments, the actuator 214 can provide a backscan resolutionon the order of 2 nm.

Although some embodiments have been discussed in terms of apiezoelectric actuator, it should be understood such that the actuatorcan be implemented using mechanical actuators, electro-mechanicalactuators, hydraulic actuators, pneumatic actuators, and the like. Thus,the term actuator 214 is not intended to denote a piezoelectricactuator, but to encompass machines that move or control a transparentplate 242 for backscanning. One of ordinary skill in the art wouldrecognize many variations, modifications, and alternatives.

In some embodiments, the actuator 214 can include one or more sensors tomonitor the motion and position of the transparent plate 242. The one ormore sensors can measure angle position, velocity, acceleration, and thelike which affect image smearing. In some embodiments the one or moresensors are capacitive sensors. In other embodiments, the one or moresensors can include a laser displacement sensor. The position of thetransparent plate 242 can be transmitted to the controller 208 and/orthe I/O module 210 for use in image processing and control loopcalculations.

The detector module 206 can include a base 212, a stage 216, and a focalplane array 218. The base 212 of the detector module 206 can be coupledto the camera body 202 and/or the controller 208. In some embodiments,the detector module 206 is communicatively coupled to the controller 208and/or the I/O module 210. In some embodiments, the controller 208 maybe positioned outside the camera body 202.

The stage 216 can include the focal plane array 218. The focal planearray 218 can be configured with one or more focal plane arrays operableto collect image data. In some embodiments, the focal plane array 218can include an infrared focal plane array detector. The an infraredfocal plane array detector can consist of an array of pixels, each pixelbeing made up of several layers. In some embodiments, the focal planearray 218 can be a visible sensor. In some embodiments, the focal planearray 218 can operate without cooling of a detector material. In otherembodiments, the detector module 206 can include thermal strappingbetween the focal plane array 218 and the base 212. In some embodiments,the focal plane array 218 can include one or more spectral filters. Insome embodiments, the focal plane array 218 can be a multi-spectral bandimager. In other embodiments, the focal plane array can include a CMOSsensor, a CCD sensor, or the like.

In some embodiments, unit cells (pixel elements) can include sub-10 μmdimensions. In some embodiments, the focal plane array 218 can includeindividual pixel elements arranged into an array such as a focal planearray that defines the detector format and image resolution. Common 4:3aspect ratio video formats include: 160×120, 320×240, 640×480, 1024×768and 1280×960. In some embodiments, the focal plane array 218 can be verylarge square format such as 4096×4096 or 8192×8192. In some embodiments,the focal plane array 218 can include a plurality of focal plane arraysas described further in FIGS. 5A and 5B. In some embodiments, thedetector module can include a bias board to provide power for the focalplane array as well as signal conditioning. In some embodiments, thedetector module 208 can include a shutter.

The dewar 232 can be any suitable type of container for enclosing thedetector module 206 and one or more additional elements in a coldenvironment. For example, the dewar 232 can be a double-walled vacuumflask with thermal-insulating properties. Additionally, embodimentsinclude a cooling device 230 within the dewar 232. The cooling device230 can operate to maintain a low temperature inside the dewar 232, andcan thereby cool the detector module 206 and/or any other elementsinside the dewar 232.

In embodiments where the detector module 206 is designed to imageinfrared light, cooling the detector module 206 can allow it to functionproperly. Additionally, it can be advantageous to lower the temperatureof lenses and other optical elements so that they do not emit backgroundnoise radiation. As mentioned above, it is advantageous in someembodiments to position the transparent plate 242 near an exit pupil sothat a small transparent plate 242 can be used. Additionally, it isadvantageous to place the transparent plate 242 outside of the dewar 232so that the temperature and stability of the components inside the dewar232 are not disturbed by the transparent plate 242 and/or actuator 214.Thus, it is further advantageous to place one or more lenses inside thedewar 232, as this can cause the exit pupil to be located outside of thedewar 232, instead of the exit pupil being located inside the dewar 232or at the optical opening of the dewar 232 (as is common in infraredimaging systems). As a result, the transparent plate 242 can then besituated both near the exit pupil and outside the dewar 232.

The controller 208 can include one or more processors 220 and memory 222to control the focal plane array 218 and the actuator 214. Thecontroller 208 can be communicatively coupled to the focal plane array218 to provide sensor clocking and image processing of sensor datacollected by the focal plane array 218. The controller 208 can also becommunicatively coupled to the actuator 214 to provide positioningsignals to rotate the transparent plate 242 for backscanning. Thepositioning signals can be proportional to a rotation rate associatedwith the backscan.

In some embodiments, the controller 208 can determine a rotation ratethat is proportional to the aircraft or satellite ground velocity andcauses the transparent plate backscan to match the motion of an imageduring image collection. The controller 208 can include one or moresensors to determine a travel velocity of the camera body 202. Thetravel velocity of the camera body 202 can be associated with theaircraft or satellite ground velocity. The one or more sensors caninclude, for example, positioning sensors, accelerometers,magnetometers, and the like. In some embodiments, the controller 208 canbe communicatively coupled to the I/O module 210 and determine thetravel velocity of the camera body 202 based on data received from theI/O Module 210. In other embodiments, the rotation rate can bepre-programmed based on a predetermined orbit velocity, such as a lowearth orbit velocity.

After determining the travel velocity of the camera body 202, therotation rate and direction can be determined using a control algorithmsuch that the image smear caused by an image sensor with a long timeconstant can be reduced or eliminated. The control algorithm can use thetravel velocity of the camera body 202 to determine a forward platformvelocity associated with the motion of an aircraft or satellite. Thecontrol algorithm can determine a rotation rate and rotation directionfor the transparent plate that will cause backscanning at a rate thatcompensates for the forward platform velocity. In some embodiments, thetransparent plate is rotated within a plane defined by the direction ofthe platform movement.

As used herein, the controller 208 can include one or more processors,which can be implemented as one or more integrated circuits (e.g., amicroprocessor or microcontroller), to control the operation of theactuator 214 and/or the focal plane array 218. The one or moreprocessors can be implemented as a special purpose processor, such anapplication-specific integrated circuit (ASIC), which may be customizedfor a particular use and not usable for general-purpose use. In someimplementations, an ASIC may be used to increase the speed of imageprocessing. In some embodiments, the controller 208 can include one ormore field programmable gate arrays (FPGAs). The FPGAs can be configuredto process sensor data collected by the focal plane array 218. One ormore processors, including single core and/or multicore processors, canbe included in controller 208. In some embodiments, the controller 208can be outside the camera body 202. In these embodiments, the focalplane array 218 and the actuator 214 can be communicatively coupled tothe I/O module 210.

The I/O module 210 can be configured to send and receive data withexternal systems communicatively coupled to the imaging system 200. Theimaging system 200 can be positioned in a vehicle such as an airplane,satellite, and the like. The data sent and received to and from externalsystems can include velocity, position, temperature, and the like. Insome embodiments, the I/O module can transmit sensor data collected bythe focal plane array 218 and/or the controller 208 to one or moresystems on the vehicle. I/O module 210 can include device controllers,one or more modems, USB® interfaces, radio frequency transceivercomponents, a serial bus, and the like to send and receive data.

FIG. 2B illustrates an imaging system 273 configured to backscan atransparent plate by rotating the transparent plate during motion of theimaging system according to another embodiment of the present invention.In the embodiment illustrated in FIG. 2B, optical elements 240 through254 illustrated in FIG. 2A are illustrated as optical elements 240through 254 are also illustrated in FIG. 2B. In addition, other elementsillustrated in FIG. 2A can be utilized in conjunction with the opticalelements illustrated in FIG. 2B and the description provided in relationto FIG. 2A is applicable to FIG. 2B as appropriate. Optical axis 274passes through the optical center of the imaging system.

Referring to FIG. 2B, objective lens 240, which can be a large outerlens, a window 244 placed at an optical opening into dewar (now shown),a band-pass filter 246, a first lens 248, a second lens 250, a thirdlens 252, and a fourth lens 254 are illustrated. Together, the lenses(e.g., 240, 248, 250, 252, and 254) can form an inverse telephoto lenssystem. The band-pass filter 246 can be designed to be transparent for aspecific range of electromagnetic radiation. For example, in someembodiments, the band-pass filter 246 may pass some or all wavelengthsof infrared light. In one example, the band-pass filter 246 may allowwavelengths in the range of 2.7 μm-4.4 μm to pass.

In the embodiment illustrated in FIG. 2B, the transparent plate 242 andthe objective lens 240 are positioned outside the dewar (not shown).Accordingly, the set of lenses 248, 250, 252, and 254 can be referred toas a rear lens group since they are disposed inside the dewar. Asillustrated in FIG. 2B, the transparent plate 242 is positioned in frontof (i.e., optically upstream of) exit pupil 270. The positioning thetransparent plate 242 between the exit pupil 270 and objective lens 240in between the rear lens group and the objective lens enables wide anglefield of view operation in the illustrated embodiment. In FIG. 2B, exitpupil entrance pupil 270 is also the aperture stop of the imagingsystem.

Although illustrated schematically as a flat or planar optical elementwith planar and parallel sides, the transparent plate 242 can havecurvature in one or both surfaces, resulting in a transparent plate thathas either positive or negative optical power.

FIG. 9 is a simplified schematic diagram illustrating a transparentplate utilized in some embodiments of the present invention. Referringto FIG. 9, imaging system 273 illustrated in FIG. 2B is shown along withadditional details related to transparent plate 242 in a particularembodiment. Transparent plate 242 includes two opposing curved surfaces910 and 912. On axis rays 920 and peripheral rays 930 are illustratedalong with the different angles of refractions associated with on axisrays 920 and peripheral rays 930, respectively. As illustrated in FIG.9, on axis rays 920 experience little refraction at the interfacesdefined by curved surface 910 and curved surface 912. For these rays,the paraxial approximation is applicable. In contrast for peripheralrays 930, illustrated by dashed lines, the high angle of incidenceresults in significant refraction at the interfaces defined by curvedsurface 910 and curved surface 912. For the high angle of incidencerays, the paraxial approximation may not apply. Accordingly, in contrastwith a planar optical element in which rays that impinge on the planartransparent plate at large angles of incidence travel a greater distancethrough the transparent plate than rays that are on-axis, thetransparent plate with curvature illustrated in FIG. 9 providesdifferent plate thicknesses as a function of the radial dimension.Accordingly, curvature of the transparent plate can vary the thicknesslaterally, introducing a prism or wedge effect and compensating for thedifferent angles of incidence (particularly at high angles of incidencein which the paraxial approximation is no longer valid, which isapplicable for the wide field of view systems discussed herein, whichmay have a field of view of 118°) and the differing optical path lengthsthat would otherwise result. Thus, differing impacts of the refractionprocess as a function of the field of view are compensated for bycurvature, contributing to the linearity in pixel shift as a function ofrotation angle discussed more fully below. Moreover, according toembodiments of the present invention, both the curvature and the tiltcan vary as a function of the position across the transparent plate.

As an example, a spherical curvature could be implemented on one or bothsurfaces of the transparent plate, for example, a curvature of 1.662 m.As will be evident to one of skill in the art, the curvature of one ormore of the surfaces of the transparent plate can vary as a function ofthe position of the transparent plate along the optical axis. Forexample, at positions closer to the objective lens, the curvature will1.465 m, whereas, at positions closer to the rear lens group, thecurvature will 1.731 m. Likewise, the rotation of the plate can vary asa function of position of the transparent plate along the optical axis.For example, at positions closer to the objective lens, the tilt will be1.876 degrees, whereas at positions closer to the rear group, the tilewill be 2.876 degrees.

Using embodiments of the present invention, substantial linearity can beprovided between angular rotation of the transparent plate 242 andshifting of pixels on the focal plane array 218 or other camera disposedat the image plane. In other words, the lateral shifting of pixels inthe image is constant or substantially constant as a function ofposition in the image for a given rotation of the transparent plate 242.Accordingly, by scanning transparent plate 242 at a constant angularrate, constant motion of camera body 202 can be compensated. Thus,linearity is provided in relation to angular rotation of transparentplate 242 and pixel position on the camera. As an example, referring toFIG. 3, as transparent plate 242 is rotated by a given amount, therelationship between pixels is maintained, preventing distortion of theimage of the object on the ground 312.

FIG. 2C illustrates an imaging system configured to backscan atransparent plate by rotating the transparent plate during motion of theimaging system according to yet another embodiment of the presentinvention. In the embodiment illustrated in FIG. 2C, optical elements240 through 254 illustrated in FIG. 2A are also illustrated in FIG. 2C.Thus, the various elements illustrated in FIG. 2A can be utilized inconjunction with the optical elements illustrated in FIG. 2C and thedescription provided in relation to FIG. 2A is applicable to FIG. 2C asappropriate.

In the embodiment illustrated in FIG. 2C, transparent plate 242 ispositioned between the rear lens group 274, i.e., the set of lenses 248,250, 252, and 254, and focal plane array 218. This position between lens254 and focal plane array 218 can be utilized in designs with sufficientworking distance and can produce a uniform shift in pixels as thetransparent plate is rotated as the ray bundles pass through thetransparent plate at different locations. Locating the transparent platebetween the focal plane array and the last lens of the rear lens group(i.e., lens 254) can result in a transparent plate with smaller physicalextent and hence smaller mass. This in turn would require lessforce/energy to drive the transparent plate than in otherconfigurations. Additionally, the ray bundles associated with each pixelelement across the focal plane array are characterized by a greaterseparation, resulting in the rays for each pixel passing equally throughthe full extent of the plate. As described herein, proper placement ofthe transparent plate enables the rays for each pixel go through adifferent region of the transparent plate, with increased separation,resulting in an increase in the amount of shift that can be obtaineduniformly.

Thus, the transparent plate 242 can be positioned at several differentpositions along the optical axis according to various embodiments of thepresent invention, including at the exit pupil as illustrated in FIG.2A, in front of the exit pupil as illustrated in FIG. 2B, or behind theexit pupil as illustrated in FIG. 2C in which the transparent plate isadjacent the focal plane array.

As described and illustrated herein, embodiments of the presentinvention provide a substantially uniform pixel shift across a widefield of view image by rotation of the transparent plate. Withoutlimiting embodiments of the present invention, the inventors believethat curvature in one or more surfaces of the transparent plate, as wellas location of the transparent plate at a position along the opticalaxis at which rays from different portions of the wide field of viewimage pass through different portions of the plate. In embodiments wherethe transparent plate is located at the exit pupil (for example, theembodiment illustrated in FIG. 2A) it may be difficult to obtain auniform shift despite curvature in the transparent plate for wide fieldof view images or images characterized by a large lateral shift inposition, however, this embodiment may provide performance withindesired target ranges for limited field of view images or limited shiftin image position. The embodiments illustrated in FIGS. 2B and 2C enablehigh levels of performance for large field of view images and/or largeimage shifts since the transparent plate is positioned at a locationalong the optical axis at which rays from different points in the scenepass through different sections of the transparent plate. With raybundles originating at different locations in the scene passing throughdifferent regions of the plate, the plate can be shaped to compensateand create a uniform shift for large field of view images and/or largeimage shifts.

FIG. 3 illustrates three successive frames captured using an imagingsystem with a transparent plate that is backscanned at a rotation ratethat matches the travel velocity of the camera body according to anembodiment of the present invention. The imaging system 300 includes adetector module 302, an actuator 304, a stage 305, a focal plane array306, and a number of optical components such as a transparent plate 342and several lenses (e.g., 340, 346, 348, 350, 352, and 354). In someembodiments, the imaging system 300 can be mounted in a moving vehiclesuch as a satellite, an aircraft, an automobile and the like. FIG. 3also illustrates the rays 310 associated with an object on the ground312.

In FIG. 3, the imaging system 300 is moving at a travel velocity 314over the object on the ground 312. In a first frame 316, the rays 310associated with the object on the ground 312 are arriving at the imagingsystem 300 at an angle X. In the example shown in FIG. 3, this angle Xis approximately 90 degrees, such that the rays 310 are arriving fromdirectly below, and such that the first position of the transparentplate 342 is oriented straight ahead (e.g., not rotated). As a result,the rays 310 are centered on the focal plane array 306 and a first image317 on the focal plane array 306 shows the object on the ground 312.

For a second frame 320, the imaging system 300 has moved relative to theobject on the ground 312 due to the velocity 314 of the imaging system300. As a result, the rays 310 associated with the object on the ground312 are now arriving at the imaging system 300 at an angle Y. As shownin FIG. 3, this angle Y is now less than 90 degrees (e.g., 89.9 degrees)such that the rays 310 arrive from a position shifted slightly to theside instead of from directly below. In the embodiment illustrated inFIG. 3, a controller, such as controller 208 described in FIG. 2A,causes the actuator 304 to rotate the transparent plate 342 (which canalso be referred to as backscanning) at a rotation rate 315corresponding to the travel velocity 314. The rotation rate 315 causesthe transparent plate 342 to rotate continuously so that the transparentplate 342 passes through a second position during the second frame. Thesecond position of the transparent plate 342 is an angle R relative tothe first position of the transparent plate 342. The rotation rate 315and angle R are chosen such that, when the rays 310 arrive at the angleY, the transparent plate 342 shifts the rays 310 to arrive at the sameposition on the focal plane array 306 as they did in the first frame316. Accordingly, a second image 321 on the focal plane array 306 is inthe same position as the first image 317 on the focal plane array 306.

For a third frame 324, the imaging system 300 has moved further relativeto the object on the ground 312 due to the travel velocity 314 of theimaging system 300. As a result, the rays 310 associated with the objecton the ground 312 are now arriving at the transparent plate 342 at anangle Z. As shown in FIG. 3, this angle Z is now less than the previousangle Y such that the rays 310 are arrive from further to the side(e.g., at 89.8 degrees). The controller causes the actuator 304 torotate the transparent plate 342 at the rotation rate 315 correspondingto the travel velocity 314 and the current angle of the transparentplate. The rotation rate 315 causes the transparent plate 342 to rotatecontinuously so that the transparent plate 342 passes through a thirdposition during the third frame. The third position of the transparentplate 342 is an angle T relative to the first position of thetransparent plate 342. The rotation rate 315 and angle T are chosen suchthat, when the rays 310 arrive at the angle Z, the transparent plate 342shifts the rays 310 to arrive at the same position on the focal planearray 306 as in the first frame 316 and the second frame 320.Accordingly, a third image 325 on the focal plane array 306 is in thesame position as the first image 317 and the second image 321 on thefocal plane array 306.

As illustrated in the first image 317, the second image 321, and thethird image 325, the drive velocity can be configured to rotate thetransparent plate 342 to stabilize the image on the focal plane array306. As a result, no image smearing occurs. If an integrated image ofthe object on the ground 312 was produced from the integration of thefirst image 317, the second image 321, and the third image 325 withbackscanning via rotation of the transparent plate 342, the integratedimage of the object on the ground 312 will have an improved signal tonoise ratio and other quality metrics in comparison to a single image oran integrated image produced from images without backscanning viarotation.

In some embodiments, the actuator 304 can rotate the transparent plate342 continuously, but with a changing rotation rate 315. For example,the rotation rate 315 can be higher when the transparent plate 342 isoriented at smaller angles (e.g., oriented directly downward or nearlydirectly downward), and the rotation rate 315 can decrease as thetransparent plate 342 is further rotated. This can be a result oftranslating the primarily linear and horizontal movement of the imagingsystem 300 relative to the ground to an angular movement of thetransparent plate 342.

Additionally, embodiments allow the transparent plate 342 to movethrough any suitable range of tilt angles, as well as start and finishrotational movement at any suitable angles. The frames in FIG. 3 showthe transparent plate 342 starting at angle of zero degrees (e.g.,facing directly downward), and then rotating to the right to an angle T(e.g., to an angle of about 75 degrees). However, the transparent plate342 could have a starting angle that is not zero degrees, and thestarting angle could be negative (e.g., to image the ground in front ofthe imaging system 300) or positive (e.g., to image the ground behindthe imaging system 300). For example, the transparent plate 342 canrotate from a starting negative angle, through a zero degree angle, andthen finish rotating at a positive angle. Additionally, the transparentplate 342 can be accelerated into the appropriate rotation rate beforeimaging begins.

Further, the light rays 310 shown in FIG. 3 may not be representative ofthe entire field of view being imaged. Instead, the light rays 310 mayonly be representative of the nadir section of the image. The totalfield of view being imaged can be larger than shown, such as a 120degree viewing angle, thereby providing a field of view of 120 degrees.

Although the description in FIG. 3 utilizes the optical layoutillustrated in FIG. 2A, in which the transparent plate 342 is positionedat or near an exit pupil, embodiments of the present invention are notlimited to this implementation and other optical layouts can beutilized. For example, the optical layout illustrated in FIG. 2B inwhich transparent plate 242 is positioned between the objective lens andthe exit pupil, as well as the optical layout illustrated in FIG. 2C inwhich the transparent plate 242 is positioned adjacent the focal planearray and optically downstream of the rear lens group can be utilizedaccording to embodiments of the present invention. Thus, a variety ofpositions of the transparent plate are possible using embodiments of thepresent invention. Moreover, embodiments of the present invention arenot limited to the use of a single transparent plate to backscan theimage via rotation of the transparent plate and multiple transparentplates working in concert, each being rotated an appropriate angularrate, which can be different for each of the multiple transparentplates, can be utilized to accomplish the image backscanning illustratedherein. One of ordinary skill in the art would recognize manyvariations, modifications, and alternatives.

It should be noted in reference to FIG. 3 that although the examplesillustrated and discussed in FIG. 3 are based on a change in the angleof the transparent plate that is referenced to a plane parallel to thefocal plane array 306 and normal to the optical ray bundle, i.e., theplane orthogonal to the optical axis 274 illustrated in FIG. 2B, this isnot required by the present invention. In addition to operation relativeto or with respect to the plane parallel to the focal plane array, thetransparent plate could be rotated relative to other planes. Moreover,the transparent plate could be rotated in the opposite direction to thatillustrated in FIG. 3, such that the transparent plate would be rotatedfrom a negative to a positive angle change to shift the image in theplane of the focal plane array. One of ordinary skill in the art wouldrecognize many variations, modifications, and alternatives.

FIG. 4A illustrates two full cycles of capturing consecutive imageframes with a transparent plate rotating to achieve a backscan relativeto the travel velocity of the camera body according to an embodiment ofthe present invention. The plot 400 illustrates a sawtooth motionprofile of the rotational movement of the transparent plate generated bythe control algorithm. In some embodiments, the control algorithmautomatically adjusts the rotation rate and rotation direction of thetransparent plate so that it corresponds to the travel velocity of theimage sensor relative to the earth's surface. The plot 400 shows thetransparent plate angle position 402 along the y-axis and the time 404along the x-axis. A starting position 406 is at zero degrees (or at anyother suitable angle) and a final position 408 is in a position based ona rotational range of the transparent plate and a length of the snapperiod. In some embodiments, the final transparent plate angle positionis determined by the shorter of the rotational range and the length ofthe snap period.

FIG. 4A includes three snap periods, a first snap period 410, a secondsnap period 440, and a third snap period 460. The first snap period 410includes capturing a first frame 413 when the transparent plate ismoving from a first angle position 412 to a second angle position 414, asecond frame 415 when the transparent plate is moving from the secondangle position 414 to a third angle position 416, and a third frame 417when the transparent plate is moving from the third angle position 416to a fourth angle position 418. The frames are captured during a totalexposure time 424 associated with the first snap period 410, which has atotal movement cycle period of 420. The exposure duration for each framedepends on the saturation time of the image sensor. In an exampleembodiment, the frames can be captured at a rate of 30 frames persecond. The resulting maximum exposure time is 33.3 milliseconds perframe, or 100 milliseconds for the three frame total exposure time.

The rotation rate of the transparent plate during the first snap period410 can be divided into three time segments. The first time segment 422is associated with a period of time for the actuator to accelerate andcause the transparent plate to reach a determined initial velocityand/or starting angle. The starting angle can be an angle at which thenext intended ground image area is in view and stationary on the focalplane.

The second time segment 424 is associated with a period of time wherethe actuator is causing the transparent plate to tilt at a rotation rate(or a range of rotation rates) corresponding to the velocity of thecamera body and/or the angle position of the transparent plate. Therotation rate opposes the motion of the image plane caused by the travelvelocity of the camera body. The velocity of the camera body cancorrespond to motion of a platform in which the camera body is placed ormounted. A platform can include, for example, a satellite, an airplane,and the like. During the second time segment 424, an image is stabilizedon the focal plane and frames can be stacked together with no imagesmear or blur. In some embodiments, if the image sensor will not besaturated, a single, continuous frame can be captured for the durationof the second time segment 424. In some embodiments the sensor canoperate at a higher frame rate and more than 3 frames can be stackedduring the second time segment 424.

The third time segment 426 is associated with a period of time requiredfor the actuator to move the transparent plate from the final angleposition 418 back to the starting angle position 406. In someembodiments, the third time segment can be considered the reset time.

The second snap period 440 includes capture of a first frame 443 whenthe transparent plate is moving from 442 to 444, a second frame 445 whenthe transparent plate is moving from 444 to 446, and a third frame 447when the transparent plate is moving from 446 to 448. The frames arecaptured during a total exposure time 454 associated with the secondsnap period 440, which has a total movement cycle period of 450.

The rotation rate of the transparent plate during the second snap period440 can be divided into three time segments. The first time segment 452is associated with a period of time for the actuator to accelerate andcause the transparent plate to reach a determined initial velocityand/or starting angle. The starting angle can be an angle at which thenext intended ground image area is in view and stationary on the focalplane. The second time segment 454 is associated with a period of timewhere the actuator is causing the transparent plate to rotate at arotation rate (or a range of rotation rates) corresponding to thevelocity of the camera body and/or the angle position of the transparentplate. During the second time segment 454, an image is stabilized on thefocal plane and frames can be stacked together with reduced or no imagesmear or blur. The third time segment 456 is associated with a period oftime required for the actuator to move the transparent plate from thefinal angle position 418 back to the starting angle position 406.

For simplicity, the third snap period 460, as shown in FIG. 4A, isshortened but includes three stacked frames similar to the first snapperiod 410 and the second snap period 440. The rotation rate of thetransparent plate during the second time segments 424 and 454 cancorrespond to a current transparent plate position and/or a travelvelocity associated with the controller 208 described in FIG. 2A. Insome embodiments, a stabilization period can be inserted between thethird time segment 426 of first snap period 410 and the first timesegment 452 of the second snap period 440.

FIG. 4B illustrates the overlap between snaps according to an embodimentof the present invention. Snap 1, snap 2, and snap 3 are shown along thedirection of travel along the ground 470. The first snap overlap 472 andthe second snap overlap 474 provide for a continuous strip imageassociated with one or more focal plane arrays on a focal plane. A fieldof view of the imaging system captures an image with a dimension 476, d,that corresponds to a distance parallel to the direction of travel alongthe ground 470. The time for a trailing edge 478 of snap 1 to travel aacross a scene captured by snap 1 and reach a first edge 480 of thefirst snap overlap 472 corresponds to the time available for theactuator to reset the transparent plate to the starting position 406.Each snap can include three frames and can use frame stacking to improvethe signal to noise ratio of the resulting snap. White noise from thedetector array improves with multiple frames added together; the SNRbenefit corresponds to the square root of the number of frames stacked.Longer duration backscanning of the transparent plate or operating thesensor with a higher frame rate results in a greater number of framesper snap and improved SNR of the resulting snap image.

FIG. 5A illustrates a focal plane array configuration consisting of 5stagger butted focal plane arrays on a stage according to an embodimentof the present invention. The imaging system 500 includes a focal planearray configuration 502, a first staring focal plane array 504, a secondstaring focal plane array 506, a third staring focal plane array 508, afourth staring focal plane array 510, and a fifth staring focal planearray 512, and thermal strapping 514. Each focal plane array can includeone or more spectral filters. In some embodiments, the focal plane arrayconfiguration 502 can be coupled to a stage as described in FIG. 2A. Insome embodiments, each focal plane array can capture images according tothe cycles illustrated in FIGS. 4A and 4B.

FIG. 5B illustrates the ground swath width 550 of a scan associated with5 stagger butted focal plane arrays according to an embodiment of thepresent invention. In some embodiments, referring to FIGS. 5A and 5B,the focal plane arrays can be staggered and butted to eliminate deadareas in a scan generated by focal plane array configuration 502. Forexample, the first staring focal plane array 504 and the second staringfocal plane array 506 are aligned together and staggered from the thirdstaring focal plane array 508, the fourth staring focal plane array 510,and the fifth staring focal plane array 512. The direction of satellitemotion 516 will capture a strip of images corresponding to each focalplane array. For example, the first staring focal plane array 504 cancorrespond to a first strip 524, the second staring focal plane array506 can correspond to a second strip 526, the third staring focal planearray 508 can correspond to a third strip 528, the fourth staring focalplane array 510 can correspond to a fourth strip 530, and the fifthstaring focal plane array 512 can correspond to a fifth strip 532. Eachstrip can consist of a plurality of snaps and each snap can correspondto a plurality of frames as illustrated in FIGS. 4A and 4B. The snapdimensions perpendicular to the direction of motion form the groundswath width 550.

While FIGS. 5A and 5B illustrate an imaging system 500 with five focalplane arrays, according to some embodiments of the present invention, animaging system can include 10 focal plane arrays, with up to 12 spectralfilters. Other embodiments can include many focal plane arrays with manyspectral filters. Moreover, a single focal plane array can be utilizedin some embodiments, accordingly, five focal plane arrays are notrequired and a fewer number can be utilized. One of ordinary skill inthe art would recognize many variations, modifications, andalternatives.

FIG. 6A provides an example illustration of a transparent plate 642shifting an incident light ray. The transparent plate 642 can haveparallel outer surfaces, and can have an index of refraction that isdifferent that the immediate surroundings. As a result, an incidentlight ray can be refracted upon entering and exiting the transparentplate 642, the entering and exiting angles can be the same. Thus, thetransparent plate 642 can shift an incident light ray to the sidewithout changing the angle or direction of the light ray. This can thencause the light ray (and any image it is part of) to shift laterally onan image sensor at the end of the optical path. Further tilting thetransparent plate 642 can cause the optical path length of the incidentlight ray to further increase.

As shown in FIG. 6A, the transparent plate 642 can have a width w, andthe transparent plate 642 can be tilted to have a transparent plateangle θ (e.g., as measured from the plane that is parallel to thedetector). A light ray can be incident upon the transparent plate 642with a light ray angle θ (e.g., as measured from an axis that is normalto the plane of the detector). The air (or other material) around thetransparent plate 642 can have a first index of refraction n₁ while thetransparent plate 642 has a second index of refraction n₂. If theindices of refraction were the same, the lateral distance that the lightray would have traveled while within transparent plate 642 is notated asat d₁. The actual lateral distance that the light ray travels withwithin the transparent plate 642 (due to internal refraction) is notatedas at d₂. Accordingly, the lateral shift of the light ray caused by thetransparent plate 642 is d₂-d₁. This distance can be defined in terms ofthe other parameters by using Snell's law. For example, the lateralshift distance of the light ray caused by the transparent plate 642 canbe described as:

${d_{2} - d_{1}} = {{w\left( \frac{n_{1}}{n_{2} - 1} \right)}{\sin \left( {\varnothing + \theta} \right)}}$

In another embodiment, the lateral shift distance can be described for arotating transparent plate. In such a scenario, Ø₁ can represent thestarting angle of the transparent plate 642 and Ø₂ can represent theending angle of the transparent plate 642. In this case, the lateralshift distance of the light ray caused by the rotating transparent plate642 can be described as:

${d_{2} - d_{1}} = {w\left\lfloor {{\left( {{\tan \left( {\varnothing_{2} + \theta} \right)} - \frac{n_{1}{{Sin}\left( {\varnothing_{2} + \theta} \right)}}{{\left( {n_{2}^{2} - {n_{1}^{2}{{Sin}\left( {\varnothing_{2} + \theta} \right)}^{2}}} \right)^{.5}n_{2}} - 1}} \right){\cos \left( \varnothing_{2} \right)}} - {\left( {{\tan \left( {\varnothing_{1} + \theta} \right)} - \frac{n_{1}{{Sin}\left( {\varnothing_{1} + \theta} \right)}}{\left( {n_{2}^{2} - {n_{1}^{2}{{Sin}\left( {\varnothing_{1} + \theta} \right)}^{2}}} \right)^{.5}}} \right){\cos \left( \varnothing_{1} \right)}}} \right\rfloor}$

FIG. 6B illustrates an example of the difference in lateral shift causedby a transparent plate between a first rotated position and a secondrotated position. As discussed in relation to FIG. 6A, for a tilt angleof ϕ₁, a lateral shift of d₁ is produced. As the tilt angle is increasedto an increased tilt angle of ϕ₂, an increased lateral shift of d₂ isproduced. Thus, FIG. 6B provides an illustration of the lateral shiftdistance achieved by rotating the transparent plate at increasing anglesas illustrated by the equation above.

FIG. 7 is a simplified flowchart illustrating a method 700 of rotating atransparent plate to backscan an image during image collection,according to an embodiment of the present invention. At step 710,provide a body. In some embodiments the body is in motion. At step 712,provide an actuator coupled to the body. In some embodiments theactuator can be a piezoelectric actuator. At step 714, provide atransparent plate coupled to the actuator. At step 716, provide a focalplane array coupled to the stage. The focal plane array can include oneor more individual focal plane arrays. At step 718, determine a bodyvelocity corresponding to motion of the body. At step 720, determine arotation rate proportional to the body velocity and/or transparent plateangle position. At step 722, rotate the transparent plate at a rotationrate (e.g., dynamic or static) relative to the motion of the body. Atstep 724, capture one or more frames during rotation of the transparentplate. In some embodiments, the one or more focal plane arrays cancapture the one or more frames.

It should be appreciated that the specific steps illustrated in FIG. 7provide a particular method of rotating a transparent plate to backscanan image during image collection, for example, during earth observationimaging, according to an embodiment of the present invention. Othersequences of steps may also be performed according to alternativeembodiments. Moreover, the individual steps illustrated in FIG. 7 mayinclude multiple sub-steps that may be performed in various sequences asappropriate to the individual step. Furthermore, additional steps may beadded or existing steps may be removed depending on the particularapplications. One of ordinary skill in the art would recognize manyvariations, modifications, and alternatives.

FIG. 8 is a simplified flowchart illustrating a control algorithm 800for rotating a transparent plate according to an embodiment of thepresent invention. At step 810, initiate motion of a camera body. Insome embodiments, the motion of the camera body can be associated with apath along the surface of the earth. The camera body can include one ormore lenses, a transparent plate, an actuator, a stage with a focalplane array, a controller, and/or any other suitable elements. In someembodiments, the camera body can include an I/O module. At step 812,send a control signal to the actuator to start a rotation of thetransparent plate at a rotation rate (e.g., static or dynamic) and in arotation direction. In some embodiments, the control signal can includea command to begin capturing images. In some embodiments, the controlsignal can start/restart a timer associated with the rotation of thetransparent plate. At step 814, read body velocity data. Body velocitydata can be determined and or received by a processor in the controller.The body velocity data can be read by the processor in the controller.In some embodiments, the body velocity data can indicate that a startingrotation rate and/or starting transparent plate angle has been reachedand the controller will send a command to the focal plane array to begincapturing one or more images.

At step 816, update the rotation rate according to the body velocity,the current transparent plate angle, and a gain coefficient. In someembodiments the gain coefficient can be a vector or matrix with multipleterms. The gain coefficient can adjust the rotation rate based on theproperties of the image sensor such as image sensor dimensions, actuatorcharacteristics, and focal plane array characteristics. In someembodiments, the gain coefficients can be applied at specifictransparent plate angle positions during backscanning via rotation. Insome embodiments, the gain coefficients can compensate for hysteresiseffects in a piezoelectric actuator to improve rotation rate smoothness.In some embodiments, additional velocity scale factors can be added toaddress variables specific to a particular implementation.

At step 818, determine the transparent plate angle position. In someembodiments, a processor in the controller can read data from one ormore transparent plate position sensors to determine the current angleof the transparent plate. In other embodiments, the transparent plateangle position can be estimated using (e.g., integrating) the rotationrate. In other embodiments, the transparent plate angle position can beextrapolated based on a predetermined time period. At step 820,determine the transparent plate reaches a cutoff angle. The cutoff anglecan be associated with a maximum time at the rotation rate or a maximumangle position of the transparent plate and actuator relative to thebody. In some embodiments, an extrapolated transparent plate angleposition can be used to determine the transparent plate will reach thecutoff angle within the predetermined time period. At step 822, afterreaching the cutoff angle, return the transparent plate and actuator toan initial position or state.

It should be appreciated that the specific steps illustrated in FIG. 8provide a particular method of controlling rotation of a transparentplate during earth observation imaging according to an embodiment of thepresent invention. Other sequences of steps may also be performedaccording to alternative embodiments. Moreover, the individual stepsillustrated in FIG. 8 may include multiple sub-steps that may beperformed in various sequences as appropriate to the individual step.Furthermore, additional steps may be added or existing steps may beremoved depending on the particular applications. One of ordinary skillin the art would recognize many variations, modifications, andalternatives.

Embodiments of the invention provide a number of advantages. Forexample, in some embodiments, a transparent plate can be used to correctfor image smearing in high-velocity imaging systems, such as satellitesand airplanes. The transparent plate can be rotated to backscan an imagearea, such that an image remains stationary on a light detector. It isadvantageous to backscan using a transparent plate instead of mirrors,as a transparent plate can be a much smaller and discrete piece ofequipment, and the total imaging system can become more compact. Thus,embodiments improve the efficiency of imaging systems by reducing theweight, size, and power needs of backscanning components. Reducing theweight of moving components also reduces the inertia created duringbackscan, which in turn reduces disturbances to the imaging system andnoise in the image. Thus, the quality of images can be advantageouslyimproved.

Further, embodiments allow some lenses and other optical elements to beplaced inside a cold shielded area (e.g., a dewar), which cools theelements and reduces their emissions of background radiation. This alsocan result in an exit pupil being moved from a dewar optical opening sothat it now is positioned away from the dewar. As a result, thetransparent plate can be placed at or near the exit pupil, or betweenthe exit pupil and the dewar. Since the light ray bundle has thesmallest diameter at the exit pupil, a small transparent plate can besufficient for shifting the light ray bundle. Thus, the backscanninghardware can be even further reduced in size.

The above embodiments primarily utilized a transparent plate that isflat. However, other embodiments allow the transparent plate to havesome curvature. This can be done for a number of reasons describedbelow.

As described above with respect to FIG. 6A, a flat plate can cause alateral shift in an incoming light path. The amount of lateral shiftwhen the plate is rotated can be dependent on the incoming angle of thelight ray. Light rays that arrive normal (e.g., at a 90 degree angle)will undergo the smallest lateral shift when the plate is rotated,slightly smaller angles (e.g., slightly less than 90 degrees) canundergo slightly more lateral shift, and even smaller angles can undergoa much larger lateral shift. Accordingly, for imaging system with largefields of view where light rays arrive from different angles at the sametime, a rotating flat plate could cause light rays at the edge of fieldof view to shift more. For example, in an optical system where lightrays incident on the transparent plate vary from normal to 18 degrees,the variation in shift can across the field of view can be about 18%. Asa result, a flat plat can cause image distortion. Thus, while a flatplate can be suitable for narrow fields of view, a flat plate can causedistortion for large fields of view.

Additionally, as the transparent plate tilts, the different light raysacross the field of view can interact with the other lenses in thesystem in a non-uniform manner.

Embodiments of the invention can mitigate and/or eliminate the variationin shift by introducing a small spherical concave curve to the frontsurface of the transparent plate and a convex curve to the rear surfaceof the transparent plate as discussed in relation to FIG. 9.

The transparent plate could alternatively be moved closer to the focalplane (e.g., in between the focal plane array 218 and optical element254) as discussed in relation to FIG. 2C to achieve the same effect.However, this may involve placing the transparent plate inside the dewar(i.e., cold box) along with cooling of the mechanism to hold thetransparent plate in the case of an infrared detector.

With the correct combination of transparent plate curvature andlocation, the lateral shift across the image field of view can besimilar within 10% of a pixel size or less. Accordingly, the lateralshift of the image across the field of view can be uniform ornear-uniform during rotation, thereby avoiding distortion and making theabove-described backscan achievable.

It can be advantageous to use a transparent plate material that has ahigh index of refraction (or a greater differential in index ofrefraction relative to the surrounding air or other surroundingmaterial). A higher index of refraction can result in a larger lateralshift. Additionally, a higher index of refraction can further reduce thevariation in shift due to angle of incident light.

FIG. 10 shows a comparison of relative edge response (RER) for variousfocal plane array configurations according to embodiments of the presentinvention. The plot 1000 illustrates the RER for an image as a functionof the distance from the edge. The x-axis 1010 illustrates the distancein pixels from an edge at zero on the x-axis 1010. The y-axis 1012illustrates the RER amplitude. A first focal plane array configurationis a static focal plane array illustrated by trace 1016 that captures asingle frame. The RER of the static focal plane array at the zero pixelis 0.52759.

A second focal plane array configuration is a non-backscanning focalplane array illustrated by trace 1014. The second focal plane arrayconfiguration is mounted on a test vehicle that simulates motion of thenon-backscanning focal plane array at a ground speed of 280 kts. The RERof the non-backscanning focal plane array travelling at a ground speedof 280 kts at the zero pixel is 0.34141.

A third focal plane array configuration is a backscanning focal planearray configuration illustrated by trace 1018 mounted on a test vehiclethat simulates motion of the focal plane array at a ground speed of 280kts. The backscanning focal plane array can be implemented bybackscanning an image via rotation of a transparent plate during motionof the imaging system as described herein. The RER of the backscanningfocal plane array at the zero pixel is 0.52953. The plot 1000illustrates that the RER of the non-backscanning focal plane array isdegraded 1020 by ˜35% from the static focal plane array. The degradedRER is due to smearing caused by the motion of the focal plane arrayduring the focal plane array integration time. The plot 1000 shows theRER of the backscanning focal plane array nearly equals the RER of thestatic focal plane array as the motion of the focal plane array iscompensated for by backscanning via rotation of the transparent plate.

Another way in which the RER can be analyzed is by considering thelinearity associated with uniformity of the pixel shift across an imagecaptured at the focal plane array. As a standard, an image captured witha static focal plane array is used. Each pixel in the image ispositioned at a pixel position. In an example embodiment, the pixelposition in the backscanned image, compared to the initial pixelposition in the static image, can have a registration error less than10% of the pixel dimension across the entire image. As an example, ifthe focal plane array is a 640×480 array with a 12 μm pitch, less than10% registration error would mean that each pixel in the image would bepositioned less than 1.2 μm from the initial pixel position. In otherwords, each pixel will have a shift of less than 1.2 μm from the initialpixel position across the entire image. Within the bounds of thismetric, a center pixel may have a registration error of zero, pixelshalf the distance from the center to the edge may have a registrationerror of 0.5 μm, and edge pixels may have a registration error of 1.0μm. Thus, although the registration error may be different amounts atdifferent portions of the image, the deviation for any given pixel willbe less than 10% of the pixel dimension. In other embodiments, thispixel registration error can be less than 9%, less than 8%, less than7%, less than 6%, less than 5%, less than 4%, less than 3%, less than2%, or less than 1%.

In some embodiments, the controller 208 can determine an angularrotation velocity that is proportional to the aircraft or satelliteground velocity and causes the rotation of the transparent platen andthe resulting backscan of the image to match the motion of the platformvelocity and/or the image during image collection. The controller 208can include one or more sensors to determine a velocity of the camerabody 202. The velocity of the camera body 202 can be associated with theaircraft or satellite ground velocity. The one or more sensors caninclude, for example, positioning sensors, accelerometers,magnetometers, and the like. In some embodiments, the controller 208 canbe communicatively coupled to the I/O module 210 and determine thevelocity of the camera body 202 based on data received from the I/OModule 210. In other embodiments, the drive velocity can bepre-programmed based on a predetermined orbit velocity, such as a lowearth orbit velocity.

As discussed, certain embodiments can be implemented in a satellitesystem, which can be relatively stable such that the stage velocitymatches the platform velocity. Alternatively, embodiments can beimplemented on aircraft, which may have slight deviations in stagevelocity. Accordingly, a feedback loop can be implemented such that therotation velocity is periodically updated on a real-time basis.Different techniques are possible such as correlating pixels betweenimages or performing the process described with respect to FIGS. 7 and9.

A computer system will now be described that may be used to implementany of the entities or components described herein. Subsystems in thecomputer system are interconnected via a system bus. Additionalsubsystems include a printer, a keyboard, a fixed disk, and a monitorwhich can be coupled to a display adapter. Peripherals and input/output(I/O) devices, which can couple to an I/O controller, can be connectedto the computer system by any number of means known in the art, such asa serial port. For example, a serial port or external interface can beused to connect the computer apparatus to a wide area network such asthe Internet, a mouse input device, or a scanner. The interconnectionvia system bus allows the central processor to communicate with eachsubsystem and to control the execution of instructions from systemmemory or the fixed disk, as well as the exchange of information betweensubsystems. The system memory and/or the fixed disk may embody acomputer-readable medium.

As described, the inventive service may involve implementing one or morefunctions, processes, operations or method steps. In some embodiments,the functions, processes, operations or method steps may be implementedas a result of the execution of a set of instructions or software codeby a suitably-programmed computing device, microprocessor, dataprocessor, or the like. The set of instructions or software code may bestored in a memory or other form of data storage element which isaccessed by the computing device, microprocessor, etc. In otherembodiments, the functions, processes, operations or method steps may beimplemented by firmware or a dedicated processor, integrated circuit,etc.

Any of the software components or functions described in thisapplication may be implemented as software code to be executed by aprocessor using any suitable computer language such as, for example,Java, C++ or Perl using, for example, conventional or object-orientedtechniques. The software code may be stored as a series of instructions,or commands on a computer-readable medium, such as a random accessmemory (RAM), a read-only memory (ROM), a magnetic medium such as ahard-drive or a floppy disk, or an optical medium such as a CD-ROM. Anysuch computer-readable medium may reside on or within a singlecomputational apparatus, and may be present on or within differentcomputational apparatuses within a system or network.

While certain exemplary embodiments have been described in detail andshown in the accompanying drawings, it is to be understood that suchembodiments are merely illustrative of and not intended to berestrictive of the broad invention, and that this invention is not to belimited to the specific arrangements and constructions shown anddescribed, since various other modifications may occur to those withordinary skill in the art.

As used herein, the use of “a”, “an” or “the” is intended to mean “atleast one”, unless specifically indicated to the contrary.

What is claimed is:
 1. An imaging system, comprising: a body; a stagecoupled to the body; a focal plane array including one or more detectorsand coupled to the stage; a lens assembly including an objective lensand a rear lens group, wherein the lens assembly is coupled to the bodyand optically coupled to the focal plane array; a transparent platecoupled to the body and optically coupled to the objective lens and thefocal plane array, wherein the transparent plate is disposed between theobjective lens and the focal plane array; and an actuator coupled to thetransparent plate and configured to rotate the transparent platerelative to an optical axis of the imaging system.
 2. The imaging systemof claim 1 wherein the transparent plate is disposed between theobjective lens and the rear lens group.
 3. The imaging system of claim 1wherein the transparent plate is disposed between an exit pupil of theimaging system and the objective lens.
 4. The imaging system of claim 1wherein the transparent plate is disposed at a location within the rearlens group.
 5. The imaging system of claim 1 wherein the transparentplate is disposed between the rear lens group and the focal plane array.6. The imaging system of claim 1 wherein the actuator is furtherconfigured to move the transparent plate in one or more directionsrelative to the focal plane array.
 7. The imaging system of claim 1further comprising: a controller coupled to the actuator andcommunicatively coupled to the one or more detectors, the controlcomprising a processor and a computer readable medium, the computerreadable medium comprising code, that when executed, causes theprocessor to: determine a travel velocity of the focal plane array;cause the actuator to rotate the transparent plate relative to theoptical axis at a rotation rate corresponding to the travel velocity ofthe focal plane array; and cause the one or more detectors to captureimage data while the actuator rotates the transparent plate.
 8. Theimaging system of claim 7 wherein the travel velocity of the focal planearray corresponds to a travel velocity of at least one of an aircraft ora satellite.
 9. The imaging system of claim 1 further comprising a dewarcoupled to the body, wherein the dewar comprises an optical aperturethat is optically aligned with the optical axis, the focal plane arrayincluding the one or more detectors, wherein the rear lens group and thefocal plane array including the one or more detectors are disposedinside the dewar.
 10. The imaging system of claim 9 wherein thetransparent plate is disposed between the objective lens and the opticalaperture.
 11. The imaging system of claim 1 wherein one or more surfacesof the transparent plate are characterized by a predetermined curvatureand the transparent plate is characterized by a non-zero optical power.12. A method comprising: determining a travel velocity corresponding tomotion of a body of an imaging system; determining, based on the travelvelocity, a movement rate for a transparent plate of the imaging system,the transparent plate being optically coupled to an image sensor of theimaging system and a lens assembly of the imaging system; sending afirst control signal to an actuator to move the transparent plate at thedetermined movement rate; sending a second control signal to the imagesensor to capture one or more frames while the actuator moves thetransparent plate; determining that the transparent plate reaches acutoff angle; and thereafter, sending a third control signal to resetthe transparent plate to an initial position.
 13. The method of claim 12wherein the movement rate comprises a rotation rate, and wherein theactuator rotates the transparent plate.
 14. The method of claim 13further comprising determining an angle position of the transparentplate using the rotation rate and a timer.
 15. The method of claim 13wherein the rotation rate is variable based on an angular position ofthe transparent plate.
 16. The method of claim 12 wherein determiningthe travel velocity includes reading travel velocity data from a memoryor receiving the travel velocity from an I/O subsystem.
 17. The methodof claim 12 further comprising: receiving position sensor data from oneor more transparent plate position sensors; and determining an angleposition of the transparent plate based on the position sensor data. 18.The method of claim 12 wherein the image sensor is disposed inside adewar, the transparent plate is disposed outside the dewar, and thetransparent plate, the image sensor, and the lens assembly are opticallycoupled to an optical opening of the dewar.
 19. The method of claim 18wherein: the lens assembly comprises an objective lens that is disposedoutside the dewar and a rear lens group disposed inside the dewar; andthe lens assembly is characterized by an exit pupil, wherein thetransparent plate is disposed between the objective lens and the exitpupil.
 20. The method of claim 12 wherein one or more surfaces of thetransparent plate are characterized by a predetermined curvature and thetransparent plate is characterized by a non-zero optical power.